Investigation of corrosion behavior of polypyrrole-coated Ti using dynamic electrochemical impedance spectroscopy (DEIS)

Abstract

In the present work, dynamic electrochemical impedance spectroscopy (DEIS) was used to investigate the corrosion behavior of polypyrrole (PPy)-coated titanium (Ti) in simulated body fluid (SBF) solution. The deposition of PPy on Ti was carried out using the cyclic voltammetry (CV) technique. The formation of the PPy coating was confirmed by infrared and Raman spectroscopy. Scanning electron microscopy (SEM) studies revealed the deposition of a 7 μm-thick PPy coating with a cauliflower-like morphology. The surface roughness and wettability of the coating were confirmed by atomic force microscopy (AFM) and water contact angle studies. The corrosion behavior of uncoated and PPy-coated Ti was investigated in SBF using DEIS and polarization studies. DEIS was carried out from the open circuit potential (OCP) to dissolution regions of the uncoated and PPy-coated Ti with a step potential of 0.5 V. The variation in the charge transfer resistance (R_ct) value as a function of potential exhibited distinctive impedance behavior for the uncoated and coated substrates. The PPy-coated Ti showed a higher R_ct at each potential thereby indicating the higher corrosion resistance of the coating. The highest polarization resistance (R_p) and lowest corrosion current density (i_corr) of PPy-coated Ti obtained from the potentiodynamic polarization studies revealed the enhanced corrosion resistance. The potentiostatic polarization studies for the PPy-coated substrate showed a low current density value at the OCP and dissolution potential and it can be attributed to the enhanced protection ability of the coating. Additional evidence in support of the enhanced corrosion protection performance of PPy was obtained from the Bode-phase angle maximum value at the OCP and dissolution potential.

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1 Introduction

Ti metal and its alloys are extensively used in implantology, especially as orthopedic implants due to their excellent corrosion resistance in bio-fluids, and the desirable mechanical properties play very important roles in improving the quality of human life.^1–6 These excellent properties of Ti metal are attributed to the presence of a stable film of titanium dioxide (TiO₂) with small quantities of TiO and Ti₂O₃ sub-oxides. It is well known that the excellent properties of TiO₂ film are responsible for its long term stability in bio-fluids and play a key role for the biocompatibility of Ti and its implants.

Surface modification of Ti and its alloys is normally done to improve several types of properties like biological, chemical and mechanical ones to meet the desired expectations. A wide variety of techniques are available for surface modification^7–10 of Ti and its alloys, such as thermal treatment,^11,12 laser deposition,^13–15 anodizing,^16–18 spraying¹⁹ and electrochemical polymerization.²⁰ Among the available techniques, electrochemical polymerization is more preferred because it provides better control of the film thickness and uniformity in the coating.

Conducting polymers, because of their high stability and ease of synthesis are used as promising materials for many applications like in polymer light-emitting diodes (LEDs),²¹ for corrosion resistance,^22,23 thin film transistors,²⁴ electromagnetic shielding,²⁵ molecular electronics,²⁶ supercapacitors,^27,28 electrochromic devices²⁹ and sensor technology^30–34 etc.

Among the various types of conducting polymers, PPy has been used for wider applications.^35–37 PPy has been extensively used because of its attractive properties e.g. ease of preparation either by chemical or electrochemical polymerization and due to its good adherent property on to the metal thereby providing high stability at the polymer film/metal interface and more electrical contact.³⁸ PPy deposition over metal shows the property of excellent corrosion resistance due to its high stability, which helps to prevent electron exchange between the adsorbed biological species and the metal.³⁹ The lower oxidation potential (0.8 V) of pyrrole in comparison to other heterocyclic monomers may probably help in the easy formation of PPy film on active metal to provide promising anti-corrosion properties. The biocompatibility of PPy has been studied extensively and reported in the literature.^40,41 For example, PPy film has been used as protective coatings for Ti–Al–V substrates to improve osseointegration performances.⁴² Moreover, a recent study on PPy film-coated implants in experiments with animals showed promising results for in vivo and in vitro studies.⁴³ Therefore, the present investigation mainly focuses on the corrosion behavior of PPy-coated titanium in SBF solution for biomedical applications.

Electrochemical impedance spectroscopy (EIS) is a technique to investigate the phenomenon between the metal/solution interface in the electrochemical system i.e. detection and monitoring of pitting corrosion, passivation of the metal surface etc. Darowicki and co-workers^44,45 developed a new impedance method termed as DEIS. It is one of the versatile and recently used electrochemical techniques to study the degradation of metallic materials, pit initiation, propagation and passivation.^46,47 DEIS helps in the measurement of impedance, which is carried out under potentiodynamic conditions and can be used to find out the point of passivation and degradation of a protective coating on metal. Passivation and surface oxide film dissolution are the two important key factors, which can affect the biocompatibility of metal. Hence, it is essential to study the different stages involved in the corrosion behavior of Ti and its alloys or PPy coatings on them to understand the surface chemistry. However, only very few published works are available involving DEIS measurement with respect to corrosion studies of Ti and its alloys. For example, the corrosion behavior of NiTi and NiTiNb alloys in physiological medium using DEIS was studied.⁴⁸ Some other literature is also available for understanding the process involved in DEIS studies of coated materials on AZ31 magnesium alloy.⁴⁹

In this paper, an attempt has been made to evaluate the corrosion behavior of uncoated and PPy-coated Ti using the DEIS technique. In order to assess their corrosion resistance, DEIS experiments were conducted from the OCP to dissolution regions of both uncoated and PPy-coated Ti as a function of applied potential.

2 Materials and methods

2.1 Preparation of substrate

Ti sheets with a size of 25 mm × 15 mm × 2 mm were used as substrates, ground using SiC emery paper up to 1200 # grit and polished. Ti samples were cleaned with double-distilled water and sonicated with acetone. The ground sheets were chemically etched using Kroll’s reagent (25 mL H₂O + 2 mL conc. HNO₃ + 1 mL conc. HF) for 15 s. This enabled the removal of the oxide layer present on the as-received material. After etching, the samples were cleaned and dried in air at room temperature and subsequently used for electrodeposition.

2.2 Film deposition

Pyrrole (monomer) was purchased from Aldrich Chemical. It was distilled twice and stored in the dark, at 0 °C. Polymerization of pyrrole over the Ti substrate was done by cyclic voltammetry in the potential range between 0.1 and 1.2 V vs. SCE, with 20 cycles with a sweep rate of 0.05 V s⁻¹ in an aqueous oxalic acid solution (0.2 M) containing pyrrole (0.2 M)⁵⁰ with nitrogen purging. The electrochemical polymerization was carried out in a three-electrode system consisting of platinum foil and a saturated calomel electrode (SCE) as the counter and reference electrodes and Ti as the working electrode. After electrodeposition the sample was rinsed with acetone and distilled water and dried in air.

2.3 Surface characterization

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was recorded in the range between 400 and 4000 cm⁻¹ using Perkin Elmer Spectrum II, USA to confirm the PPy coating. The formed PPy coating was confirmed by Raman spectroscopy using a FT Raman, Bruker RFS 27 instrument with 1064 nm, and a Nd:YAG source was used for the analysis. Raman spectra were recorded with a 500 s data point acquisition time in the spectral range of 200–1200 cm⁻¹. To observe the morphological features, scanning electron microscopy (Hitachi Model-S 3400, Japan) with an accelerating voltage of 10–30 kV was employed. The surface topography and roughness were determined by atomic force microscopy (XE 70), model Park system, Korea. The water contact angle value for the uncoated and PPy-coated specimens was calculated with the help of UTHSHCSA image tool software using Euromex optical microscope equipment with a CCD camera.

2.4 Electrochemical characterization

All electrochemical studies of the uncoated and polymer-coated Ti samples were conducted using an electrochemical workstation, Autolab, PGSTAT-12. The electrochemical studies were carried out in the conventional three-electrode glass cell. SBF solution was used as physiological medium and its composition⁵¹ is given in Table 1. The Ti sample was used as the working electrode. Pt foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The stabilized OCP was obtained after 45 min of immersion, then the potentiodynamic polarization studies were carried out from −1.0 to 1.5 V at a scan rate of 1 mV s⁻¹. DEIS was studied using a frequency response analyzer (FRA), starting from the OCP to dissolution region with a 50 mV step potential in the frequency range between 0.01 Hz and 100 kHz with 10 mV amplitude. The potentiostatic polarization was studied at the OCP and dissolution potential for uncoated and PPy-coated Ti.

Table 1 Composition used for the preparation of SBF solution

Reagents	Amount in 1000 mL
NaCl	8.031 g
NaHCO3	0.358 g
KCl	0.223 g
K2HPO4·3H2O	0.233 g
MgCl2·6H2O	0.314 g
1.0 M HCl	39.0 mL
CaCl2	0.290 g
Na2SO4	0.074 g
(HOCH2)3CNH2	6.116 g
1.0 M HCl	Appropriate amount for adjusting the pH

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3 Results and discussion

3.1 Electrochemical deposition of PPy on Ti electrode

The PPy film was deposited by potentiodynamic polymerization using aqueous solution containing oxalic acid as the supporting electrolyte in the presence of pyrrole monomer. Fig. 1 shows the cyclic voltammogram of Ti metal in 0.2 M oxalic acid with PPy monomer. It can be observed that the addition of pyrrole monomer (0.2 M) in the presence of (0.2 M) oxalic acid solution changed the behavior of Ti metal. The increase in current density indicates that the polymerization of pyrrole starts at about 0.7 V vs. SCE. On increasing the number of scans, the current density is decreased. After completion of all cycles, a uniform thickness of the adherent black homogenous PPy film was deposited on Ti.

Fig. 1 Cyclic voltammogram of Ti metal with pyrrole monomer in oxalic acid.

3.2 ATR-FTIR and Raman spectral studies

The ATR-FTIR spectrum of the electropolymerized PPy coating on Ti is shown in Fig. 2(a). In the spectrum of PPy, the characteristic peaks at 3412 and 1074 cm⁻¹ are assigned to N–H and C–H stretching vibrations in the pyrrole ring.⁵² The peak at 1409 cm⁻¹ corresponds to C–N stretching vibration.⁵³ The peak position at 1031 cm⁻¹ is related to the N–H deformation band. The peak at 1600 cm⁻¹ corresponds to the C=C stretching vibration of the pyrrole ring. The peaks at 1641 and 1571 cm⁻¹ represent the symmetric and anti-symmetric stretching modes in the aromatic ring, respectively. The C–H out-of-plane bending vibrations are observed at 964 and 956 cm⁻¹, respectively.

Fig. 2 (a) ATR-FTIR spectrum of PPy-coated Ti and (b) Raman spectrum of PPy-coated Ti.

The Raman spectrum of the PPy coating is shown in Fig. 2(b). The strong band at 1592 cm⁻¹ represents the oxidized state of the polymer and it corresponds to the C=C stretching vibrations of PPy. The absorption peak at 1383 cm⁻¹ is assigned to the C–N stretching vibration of PPy.⁵⁴ The absorption peak at 1491 cm⁻¹ is assigned to C–C stretching vibration. This proves that PPy has been coated on Ti metal through electropolymerization. The peaks appearing at 932 cm⁻¹ and 971 cm⁻¹ are defined as ring deformation and the peak at 1080 cm⁻¹ indicates the in-plane bending vibration in the pyrrole ring.

3.3 Scanning electron microscopy studies

The SEM images of the uncoated and PPy-coated Ti and a cross-section image of PPy-coated Ti are shown in Fig. 3. The surface morphology of uncoated Ti shows the presence of grooves due to grinding and mechanical polishing. The SEM image of the PPy-coated surface reveals a cauliflower-like morphology,⁵⁵ which shows microspherical grains with variable particle sizes and seems to be compact without porosity i.e. characteristic of polypyrrole. From the cross-section image, it can be observed that the thickness of the PPy coating is 7 μm.

Fig. 3 SEM images of (a) uncoated Ti, (b) PPy-coated Ti, and (c) cross-section of PPy-coated Ti.

3.4 Atomic force microscopy studies

The topography images of the uncoated and PPy-coated Ti are shown in Fig. 4. The AFM image of the uncoated Ti was found to have a flat surface consisting of nodules appearing along the polishing direction with a lower surface roughness value of about 32.47 nm. The PPy coating shows uniform deposition along with variable grains over the surface. The AFM 3D image of the PPy coating shows a well-defined cauliflower-like pattern, which has a perfect correlation with the SEM images with a high surface roughness value of about 54.96 nm. The high roughness value of the coating influences the bone bonding ability and bioactivity. An increase in the roughness value increases the cell behaviour viz. cell attachment and osseointegration.

Fig. 4 2D and 3D AFM images of (a) uncoated Ti and (b) PPy-coated Ti.

3.5 Water contact angle studies

The surface wettability of implant material plays a crucial role in influencing the bioactivity and cell adhesion. The contact angle value depends on the surface roughness, which is also attributed to the chemical effect and surface energy of the coating. A metal surface that is hydrophilic in nature increases the biocompatibility of the implant material in SBF.⁵⁶ Fig. 5 shows the contact angle value of uncoated Ti, which was found to be 87 ± 2°, which indicates the hydrophobic nature of the material, whereas the PPy-coated Ti exhibited a lower contact angle value of about of 57 ± 2°. The decreased contact angle value of PPy-coated Ti confirmed the hydrophilicity. This may be probably due to the presence of amine groups present in the PPy polymer matrix. The hydrophilic nature of the PPy coating improves the ion exchange property of the implant material in SBF and will lead to the existence of apatite growth.⁵⁷

Fig. 5 Water contact angle images of (a) uncoated Ti and (b) PPy-coated Ti.

3.6 Potentiodynamic polarization studies

Potentiodynamic polarization curves of uncoated and PPy-coated Ti after exposure in SBF solution were obtained and are shown in Fig. 6(a). Electrochemical parameters such as i_corr (current density), E_corr (corrosion potential) and polarization resistance (R_p) are summarized in Table 2. The E_corr value of uncoated Ti was found to be low i.e. 0.496 V, due to the presence of low conducting TiO₂ film formed on the surface which is replaced by the corrosion process. In the case of the PPy coating, there is a positive shift of 0.100 V because the PPy chain carries more charge and helps to make a stable potential for the working electrode. Accordingly, the i_corr value of the PPy coating is decreased by one order of magnitude compared to that of the uncoated Ti. The value of the PPy coating is 0.088 μA cm⁻², which is lower than that of the uncoated substrate (0.14 μA cm⁻²). It may be due to the formation of the PPy coating, which contains a greater number of conjugated double bonds, and carries a high number of electrons, which helps in adsorption to the working electrode. Hence, the coating is more stable. The R_p value of the coated sample is found to be higher than that of the uncoated Ti, which confirms the increase in corrosion resistance. These results indicate that the coated sample possess better corrosion resistance than the uncoated metal. The polarization resistance of the uncoated and PPy-coated Ti was determined using the Stern–Geary equation (eqn (1)):where β_a and β_b are the anodic and cathodic Tafel slopes, i_corr is the corrosion current density and R_p is the polarization resistance.^58,59

Fig. 6 (a) The potentiodynamic polarization curves of uncoated Ti and PPy-coated Ti in SBF. (b) The anodic polarization curves of uncoated Ti and PPy-coated Ti in SBF.

Table 2 Corrosion parameters of uncoated and PPy-coated Ti obtained by the Tafel plot in SBF

Sample	Ecorr (V)	icorr (μA cm^−2)	Rp (kohm cm^2)	βa (V dec^−1)	βc (V dec^−1)
Uncoated Ti	−0.496	0.14	283.252	4.603	6.264
PPy-coated Ti	−0.397	0.088	535.998	2.708	6.309

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The dissolution behavior or onset of corrosion for uncoated and PPy-coated Ti was explained on the basis of the anodic polarization curve, which is shown in Fig. 6(b). Anodic polarization basically provides information about the passivation and dissolution behavior of the metal and metal/coating surface. From the plot, it can be observed that the increased current density for uncoated and PPy-coated Ti is around 1.2 V and 1.25 V, respectively. The reason behind this is that the degradation started at these regions for both the uncoated and PPy-coated Ti. Therefore, the corrosion resistance decreased for both substrates.

3.7 DEIS studies

In anodic polarization (Fig. 6(b)), we can see that the nature of the polarization curves of both the uncoated and PPy-coated Ti has exhibited a stable passive behavior without exhibiting an active passive behavior. The passive behavior is associated with the increased current because of the localized corrosion. From the plot, a sudden increase in the anodic current was evident for the onset of the corrosion or dissolution region at around 1.2 V for uncoated and 1.25 V for PPy-coated Ti.

Fig. 7 shows the DEIS plots of uncoated Ti as a function applied potential. At the OCP, incomplete semicircle impedance behavior was observed followed by a distinctive impedance response up to 1 V which after that decreases with increasing potential. The results show a high resistance nature at initial potentials, indicating high corrosion resistance, and the metal surface is fully covered with a stable oxide film of Ti. The decrease in the resistance value after 1 V indicated the thinning of the oxide film when the potential increased from a lower to a higher extent.⁶⁰ The film dissolution, onset of corrosion at 1.2 V, and the surface exposure in the electrolyte medium increase the corrosion rate. The appearance of this trend might be due to the adsorption of corrosive ions on the metal surface from the electrolyte solution. Comparing with the anodic polarization results, the increase in current density is also evidence for the dissolution of the passive film at 1.2 V. In order to characterize the passive film formed on Ti, an equivalent circuit is used and the fitted results showed a single time constant for all the potentials as shown in Fig. 9(a), where R_s is the electrolyte solution resistance, R₁ is the charge transfer resistance of the passive film and Q₁ is the double-layer capacitance of the passive film that gives information about the polarity and charge separation of the oxide layer/electrolyte interface. Generally in titanium oxide films, distributed relaxation phenomena are observed, and due to this reason here Q was replaced with a constant phase element (CPE) and this is written as Z_CPE = [Q(jw)ⁿ]⁻¹, where Q is the CPE constant, j is the imaginary number, ω is the angular frequency and −1 < n < 1. The n values are in the range of 0.5 to 1 and are associated with a non-uniform current behaviour due to the surface porosity and surface roughness.^61,62 If n = 1 the substrate behaves with ideal capacitive behavior. If n is less than one, the substrate behaves with non-ideal capacitive behavior and a constant phase element exists.

Fig. 7 DEIS Nyquist plots of uncoated Ti as a function of applied potential in SBF solution.

Fig. 8 shows the DEIS plots of the PPy coating in SBF. A higher magnitude of impedance was observed at lower potentials (−0.55 to 0.3 V) for the PPy coating compared to that of the uncoated Ti. This is due to the lower number of surface defects as well as surface homogeneity, which could be due to the existence of the stable coating that protects the movement of corrosive ions from the electrolyte solution. The gradual increase in the impedance values in the potential range region between 0.3 and 0.8 V is due to the conjugated double bonds and polar N–H group present in the pyrrole ring.⁶³ Consequently, there is a strong bonding interface between the metal and coating. The gradual decrease in the resistance value at higher potentials, 0.85 to 1.25 V, was attributed to the increased porosity of the coating. The coating is exposed in the electrolyte medium and the low progressive degradation ability of the coating results in a decrease in thickness, which might allow more corrosive ions to easily go to the surface leading to the breakdown of the film. This is in good agreement with the trend observed from the anode polarization with an increased current density at 1.25 V. All the spectra for the coated substrate show a high magnitude of impedance compared to that of the uncoated Ti because the charge conduction in PPy favours delocalization of charge. Hence, the coating is more stable and prevents the corrosion reaction, which needs a localization of charge. At the OCP to dissolution region, the increased charge transfer resistance (R_ct) for the PPy-coated Ti compared to that of the uncoated Ti indicated a low corrosion rate. The equivalent circuit used for the PPy-coated Ti is shown in Fig. 9(b) where R_s represents the electrolyte solution resistance between the working and reference electrodes, R₁ is the charge transfer resistance of the coating, Q₁ is the double-layer capacitance of the metal/coating interface and Q₂ is the capacitance of the coating/electrolyte interface. The existence of a higher charge transfer resistance value of the PPy-coated Ti also includes the film resistance (R₂) as shown by the equivalent circuit in Fig. 9(b). The PPy coating acts as a physical barrier between the metal and electrolyte. Distinctive impedance behavior was observed for PPy-coated Ti from the OCP to dissolution region as a function of potential. The decreased corrosion resistance at the dissolution region compared to the OCP region for the coated substrate is attributed to the coating defects that arise from corrosion reactions by the influence of a high positive potential. The circuit used for the coated substrate at the dissolution region included the elements R₂, which represents the pore resistance, and Q₂, which is the pore capacitance, and these are replaced by the coating resistance (R₁) and coating capacitance (Q₂), respectively.

Fig. 8 DEIS Nyquist plots of PPy-coated Ti as a function of applied potential in SBF solution.

Fig. 9 Equivalent circuit diagram for (a) uncoated Ti and (b) PPy-coated Ti.

The EIS parameters of the uncoated and PPy-coated Ti were obtained from the equivalent circuits and are given in Table 3. From the table values, a higher charge transfer resistance (R_ct) of the PPy coating than that of the uncoated substrate at the OCP and dissolution potential regions gives more protection by improving the corrosion resistance of Ti metal in SBF. This is attributed to the homogeneous, compact and relatively less porous surface for the PPy-coated Ti, as confirmed by SEM and AFM analysis. Therefore, the coatings showed enhanced corrosion resistance for all potentials. The double-layer capacitance values of the uncoated and PPy-coated Ti are higher in the dissolution region than in the OCP region. This is due to the adsorption of corrosive ions being more on the surface when the potential is raised to a higher extent. The capacitance of the passive film on the substrate and coating was increased in the dissolution regions because of the higher conductivity between the substrate/electrolyte and coating/electrolyte interfaces. The n values of the uncoated and PPy-coated Ti at the OCP and dissolution regions are close to ideal capacitive behavior. The higher n values of the PPy-coated Ti indicates more resistance of corrosive ion penetration and impedes the corrosion process even at high potential.

Table 3 EIS parameters for uncoated and PPy-coated Ti at the OCP and dissolution regions obtained by equivalent circuits

Potential	OCP		Dissolution
Material	Uncoated Ti	PPy-coated Ti	Uncoated Ti	PPy-coated Ti
Rs (Ω cm^2)	1.9	1.6	2.6	3.1
Q1 (S s^n cm^−2)	1.919 × 10^−5	5.653 × 10^−8	5.17 × 10^−4	4.757 × 10^−5
R1 (Ω cm^2)	1.431 × 10^5	2.626 × 10^5	5.982 × 10^4	6.509 × 10^4
n	0.89	0.94	0.76	0.83
Q2 (S s^n cm^−2)	—	6.051 × 10^−6	—	4.104 × 10^−5
R2 (Ω cm^2)	—	109.3	—	52.31
n	—	0.91	—	0.81

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Fig. 10 shows the Bode-phase angle plots of the uncoated and PPy-coated Ti at the OCP and dissolution regions. At the OCP, the phase angle plots of the uncoated and PPy-coated substrates started at almost the same regions in the high frequency region followed by reaching around −80° in the mid frequency region. After that there is a decrease in the phase angle value for the uncoated substrate in the low frequency region. In general, the low frequency region of the phase angle plots is used to determine the corrosion resistance of the material. The higher phase angle value of PPy-coated Ti in the low frequency region revealed enhanced corrosion resistance. The higher phase angle shift for PPy-coated Ti compared to that of uncoated Ti indicated the presence of a highly stable and compact coating and less interaction of corrosive ions present in SBF. At the dissolution region, the phase angle values in the low frequency region dropped to −20° for both the uncoated and PPy-coated Ti. These results showed evidence for the decrease in impedance magnitude with increasing potential. The uncoated and PPy-coated substrates behaved as a non-ideal capacitive system and there exists a constant phase element (CPE) for all the potentials because the n values observed from the CPE are less than one for all the applied potentials.

Fig. 10 Bode-phase angle plots for uncoated Ti and PPy-coated Ti metal at (a) the OCP and (b) the dissolution region in SBF solution.

Bode impedance plots for uncoated and PPy-coated Ti at the OCP and dissolution regions are shown in Fig. 11(a) and (b). In the Bode impedance plots, the high frequency region explains the local surface defects formed on the coating by corrosion, the mid frequency region represents the reaction involved within the film, and the low frequency region reveals the behavior occurring because of corrosion in the metal/film interface. At the OCP, the low frequency spectrum of uncoated Ti is quite low compared to that of the PPy-coated Ti, which revealed that the low conducting TiO₂ passive film was replaced due to the distribution of ions towards the substrate from the electrolyte solution. The high impedance was consistent with the insulating nature of the coating against corrosion which provides a greater barrier property to the metal, even when the potential increased from a lower to a higher region, which impedes the access of anions in SBF for further attack to the metal/coating interface. At dissolution regions, the decrease in the Bode impedance behavior for both the uncoated and PPy-coated Ti is related to the degradation ability by the influence of potential resulting in the increased capacitance values for both the substrates. This may be due to the increased conductivity between the substrates and electrolyte medium.

Fig. 11 EIS Bode impedance plot for uncoated Ti and PPy-coated Ti metal at (a) the OCP and (b) the dissolution region in SBF solution.

A potentiostatic polarization test was carried out for uncoated and PPy-coated Ti at the OCP and dissolution potentials as a function of time, and the results are shown in Fig. 12. At the OCP, the current density values of the uncoated and PPy-coated Ti decreased rapidly in the beginning followed by stabilization in the range of 7.13 × 10⁻⁵ (A cm⁻²) for uncoated and 7.45 × 10⁻⁵ (A cm⁻²) for PPy-coated Ti. There is no significant difference in the current density values on comparing both substrates but the slightly decreased value for PPy-coated Ti indicated increased corrosion resistance.

Fig. 12 The change in current density for uncoated and PPy-coated Ti metal at (a) the OCP and (b) the dissolution region in SBF solution.

The SEM images (Fig. 13(a) and (b)) also prove that not much corrosion occurred on the surface of the uncoated and PPy-coated Ti. Subsequently, a higher corrosion resistance of the PPy coating was related to the low passive current density during the test. At the dissolution region, the passive TiO₂ film on metal degraded gradually because of film breakdown and a greater number of corrosive ions entering the surface by the influence of high potential, resulting in the high current density value of about −1.07 × 10⁻⁴ (A cm⁻²) for uncoated Ti. The generation of a higher current density value of uncoated Ti revealed an increased corrosion rate at higher potential. These results demonstrated that the passive film formed on metal is no longer resistant by the influence of high potential stress. The SEM image of uncoated Ti (Fig. 13(c)) also supports the fact that more corrosion takes place at higher potential and decreases the protective efficiency of the material. Interestingly, there is a slight increase in the current density value for PPy-coated Ti because of the low degradation followed by stabilization at 7.187 × 10⁻⁵ (A cm⁻²). There is not much difference in the current density value in comparison with the current density observed at the OCP region. There is low reduction stability of the coating at higher potential, and the low current is due to the presence of a stable polymer film with less morphological change, which can be seen in Fig. 13(d).

Fig. 13 SEM images of corroded surfaces of (a) uncoated Ti, (b) PPy-coated Ti at the OCP, (c) uncoated Ti, and (d) PPy-coated Ti at dissolution in SBF solution.

4 Conclusions

A PPy coating was deposited successfully by the electropolymerization technique using CV. ATR-FTIR and Raman studies confirmed the deposition of PPy on Ti by the presence of N–H and C=C stretching vibrations. SEM and AFM images of the coated substrate revealed the deposition of PPy with a cauliflower morphology and its high roughness value. A low contact angle value revealed the hydrophilic nature of the coated substrate. The higher R_p and reduced i_corr values demonstrated the enhanced corrosion resistance for the PPy-coated Ti. DEIS studies confirmed the changes in the impedance value as a function of potential and the PPy coating gave more protection even at higher potentials. These results are in good agreement with the current trend observed from the polarization curves. High Bode impedance and phase angle maximum values were observed for the coated substrate at the OCP and dissolution region. The low current density value observed from the potentiostatic polarization studies confirmed the less corroded surface and hence the more protective efficiency of the PPy coating.

Acknowledgements

One of the authors, Bhavana Rikhari, is grateful to Anna University for the financial assistance under Anna Centenary Research Fellowship Scheme. The author is also very much thankful to Department of Science and Technology Fund for improvement of S & T infrastructure in higher education institutions and University Grant Commission Departmental Research Support Scheme.

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